Re: What is the point of Private?

[Simon Smith <Simon@skcs.co.uk>]

thanks for all the replies.

All beginning to make sense now :-)

chris wrote:

> It's there for two reasons.  It hides the implementation from clients of 
> the code and makes the compilers job easier.  The decision seems to be 
> that the compiler should be able to determine how much space something 
> will take at compile time*.  This means they had to put the 
> representation in the spec, so the compiler knew what to allocate.
> 
> The alternatives are to leave everything public or to put the 
> representation in the body.  Making everything public means someone can 
> adversely manipulate the contents of your data structures.  They can 
> break the rules, and mess up the software.  Completely hiding the 
> representation by putting it in the body has a different trade off.  On 
> the one hand, you can substitute any representation by changing the body 
> alone, but on the other hand the compiler won't know how big the data 
> structure is and it has to treat all things the same way (by wrapping or 
> boxing them up - which has a cost).
> 
> It's a trade off.  The Ada people chose to put it in the spec because it 
> simplified the compiler, and it best suited their over all goals.  Other 
> languages allow you to completely hide the representation in the 
> implementation.  e.g. SML, Ocaml.  Some make it public.
> 
>> Why is it there? I am sure I have heard talk that its to stop the 
>> client from seeing exactly how the data is stored/manipulated however 
>> if people really did want to know this wouldn't they just scroll down 
>> the ada spec file and find out for themselves or am I missing 
>> something here?
> 
> 
> Client code can't do that though.  This means someone who uses the 
> compiled package can't see how it's implemented, but the maintainer can.
> 
> 
> 
> Chris
> 
> *There are probably other reasons too, I don't know what they are though.